Method of photoelectrophoretic imaging



May 21, 1968 H. E. CLARK 3,

METHOD OF PHOTOELECTRQPHORETIC IMAGING Filed July 21, 1967 2Sheets-Sheet 1 FIG. IA

INVENTOR. HAROLD E. CLARK fQwM,

ATTORNEYS May 21, 1968 H. CLARK 3,

METHOD OF PHOTOELECTROPHORETIC IMAGING Filed July 21, 1967 2Sheets-Sheet 2 a3 umw A 9 QQ lllllllllllul'lll'l' NN V &

IIllllnll'lllllll'll ++++++TT++++++ M INVENTOR. HAROLD E. CLARK [M aATTORNEYS United States Patent l 3,384,566 METHOD OFPHOTOELECTROPHQRETKC IMAGING Harold E. (Ilark, Penfieid, N.Y., assignorto Xerox Corporation, Rochester, N.Y., a corporation of New YorkContinuation-impart of application Ser. No. 384,681, July 23, 1964. Thisapplication July 21, 1967, Ser. No. 655,023

32 Claims. (Cl. 204-481) ABSTRACT OF THE DISCLOSURE An electrophoreticimaging system is disclosed in which a layer of a suspension comprisingelectrically photosensitive particles in a liquid carrier is placedbetween a pair of electrodes, one of which is transparent, an electricfield is imposed across the suspension and the suspension is exposed toan image through the transparent electrode whereby an image of migratedparticles forms on at least one electrode.

Background of the invention This invention relates in general to a novelimaging system and more specifically, to an imaging system based on thephenomenon of photoelectrophoresis. This application is acontinuation-in-part of my copending application, Ser. No. 384,681,filed July 23, 1964 and now abandoned.

Although many photographic systems are known today, such as, forexample, processes based on photolytic reduction of silver salts orchromate compounds, photolysis of the ferric ion to the ferrous ion, theuse of phototropic compounds, the diazo coupling reaction, variousthermo graphic techniques, photobleaching of dyes, photopolymerization,etc., all of the known systems of photography suffer from oneshortcoming or another. For example, some require expensive and complexinitial preparation of the photosensitive media while others suffer fromdeficiencies in resolution capabilities, photographic speeds, spectralsensitivity and the like. In addition to the aforementioned shortcomingsof many of the present-day photographic systems, additional processingis generally required to produce a visible image from the latent imageproduced on the photosensitive media after its exposure to light. Inconventional silver halide systems, for example, this generally requiresdeveloping and fixing of the negatives, printing the negative on aprinting paper followed by developing, fixing and drying of the positiveprint.

Now, in accordance with the present invention, there is described animaging system in which one or more types of photosensitive radiantenergy absorbing particles which are believed to bear a charge whensuspended in a substantially nonconductive liquid carrier are suspendedin such a liquid, placed in an electroded system and exposed to animage. When these steps are completed, particle migration takes place inimage configuration providing a visible image at one or both of theelectrodes. The system employs as the principal component of theparticles pigments which are themselves photosensitive and whichapparently undergo a net change in charge polarity upon exposure toactivating radiation, by interaction with one of the electrodes. Noother significant photosensitive elements or materials are required,making for a very simple and inexpensive imaging technique. Mixtures oftwo or more differently colored particles are used to secure variouscolors of images and imaging mixes having different spectral responses.Pigments in these mixes may have either separate or overlapping spectralresponse curves and may even be used in subtractive color synthesis, as

3,384,566 Patented May 21, 1968 described in copending application Ser.No. 384,737, filed July 23, 1964.

Although imaging systems based on particle migration techniques havebeen suggested in the prior art as described, for example, in U.S.Patent 2,940,847 to Kaprelian, these systems have proven so lightinsensitive, produce such poor images and are so complex and difficultto manufacture that they have never been accepted commercially. Theseprior art systems employ complex particles including at least two andfrequently more layers of various different materials including, forexample, photoconductive cores with varying high resistivity lightfiltering overcoatings and sometimes include glass cores, encapsulateddyes and similar components, which were previously thought to benecessary to provide light filtering action, to prevent particleinteraction and oscillation in the system and perform other functions.It has, however, been discovered quite unexpectedly and surprisingly inaccordance with the present invention that this complex layer structureis not only unnecessary but is even undesirable and that instead, simpleparticles made up primarily of colored photosensitive pigments may beused to produce excellent results under the conditions describedhereinafter.

Summary of the invention Accordingly, it is an object of this inventionto define a novel and extremely uncomplicated imaging system.

An additional objective of the invention is to define a novel imagingsystem capable of direct positive imaging.

Still another objective of the invention is to define a novel imagingsystem for producing images in one or more colors.

Yet a further objective of this invention is to describe novel imagingcompositions useful in the system of the aforesaid objectives.

Brief description of the drawings The above and still further objects,features, and advantages of the present invention will become apparentupon consideration of the following detailed disclosure of theinvention, especially when taken in conjunction with the accompanyingdrawings where:

FIGS. 1 and 1a are side views of simple exemplary systems for carryingout the steps of the invention;

FIGS. 2a, 2b, 2c, and 2d are broken side diagrammatic views ofconsecutive occurrences which apparently take place during the operationof the imaging process.

The sizes and shapes of elements of the drawings should not beconsidered as actual sizes or even proportional to actual sizes becausemany elements have been purposely distorted in size or shape in order tomore fully and clearly describe the invention.

Referring now to FIGURE 1, there is seen a transparent electrodegenerally designated 11 which, in this exemplary instance, is made up ofa layer of optically transparent glass 12 overcoated with a thinoptically transparent layer of tin oxide commercially available underthe name NESA glass. This electrode shall hereafter be referred to asthe injecting electrode. Coated on the surface of injecting electrode 11is a thin layer of finely divided electrically photosensitive particlesdispersed in a substantially insulating liquid carrier. During thisinitial part of the description of the invention, the term electricallyphotosensitive may be thought of as any particle which, once attractedto the injecting electrode will migrate away from it under the influenceof an applied electric field when it is exposed to actinicelectromagnetic radiation. A detailed theoretical explanation of theapparent mechanism of operation of the invention and the electricallyphotosensitive nature of the particles is given below. The liquidsuspension 14 may also contain a sensitizer and/or a binder for thepigment particles which is at least partially soluble in the suspendingor carrier liquid as will be explained in greater detail hereinafter.Above the liquid suspension 14 is a second electrode 16 which isconnected to one side of the potential source 17 through a switch 18.The opposite side of potential source 17 is connected to the injectingelectrode 11 so that when switch 18 is closed, an electric field isapplied across the liquid suspension 14 between electrodes 11 and 16. Animage projector made up of a light source 19, a transparency 21, and alens 22 is provided to expose the dispersion 14 to a light image of theoriginal transparency 21 to be reproduced. It should be noted at thispoint that injecting electrode 11 need not necessarily be opticallytransparent but that instead, electrode 16 may be optically transparentand exposure may be made through it from above as seen in FIGURE 1.

The embodiment show in FIGURE 1a uses identical numerals to identifyidentical parts of the device and is the same as the FIGURE 1 embodimentof the invention except for the fact that electrode 16 is made in theform of a roller 23 having a conductive central core 24 connected to thepotential source 17. The core is covered with a layer of a blockingelectrode material 26, which may, for example, be baryta paper. In boththe FIGURE 1 and FIGURE 1a embodiments of the invention, the particlesuspension is exposed to the image to be reproduced while potential isapplied across the blocking and injecting electrodes by closing switch18. In the FIGURE 1a embodiment of the invention, roller 23 is caused toroll across the top surface of injecting electrode 11 with switch 18closed during the period of image exposure. This light exposure causesexposed particles originally attracted to electrode 11 to migratethrough the liquid and adhere to the surface of the blocking electrodematerial 26 leaving behind a particulate image on the injectingelectrode surface which is a positive duplicate of the originaltransparency 21. Similarly, in the FIGURE 1 emobdiment electrode 16 maybe removed, after exposure, from the surface of the pigment suspension14 whereupon the relatively volatile carrier liquid evaporates offleaving behind the particulate image. This image may then be fixed inplace, as for example, by placing a lamina tion over its top surface orby virtue of a dissolved binder material in the carrier liquid such asparaffin wax or other suitable binders that come out of solution as thecarrier liquid evaporates. About 36% by weight of parafiin binder in thecarrier has been found to produce good results. The carrier liquiditself may be molten paraflin wax or other suitable binder in a liquidstate which is selffixing upon cooling and return to the solid state. Inthe alternative, the image remaining on the injecting electrode may betransferred to another surface and fixed thereon.

FIGURES 2a through 2d show in detail a proposed theoretical operatingmechanism for the system with the particle size and carrier liquidthickness greatly exaggerated for purposes of illustration. Since thesystem has been experimentally shown to be operative, there is, ofcourse, no intention to limit the invention to this theory of operationwhich is only given for clarification. In these figures, identicalnumerals have been used to identify parts of the system which areidentical with those in FIG- URES 1 and 1a. Referring now to FIGURE 2a,it is seen that the particle dispersion generally identified as 14consists of the substantially insulating carrier liquid 27 havingcharged particles 28a, 28b, 280, etc., suspended therein. The particles28 bear a net electrostatic charge when suspended in the carrier liquid27 which is believed to be related to the triboelectric relationship ofthe particles and liquid. The charges are trapped or bound either withinthe body of the particles or at their surfaces. The net charge on theparticles may be either positive or negative; however, in this instance,an encircled negative charge in each particle has been employed todiagrammatically indicate that trapped negative charge carriers givethat particular particle a net negative electrostatic charge. Whenswitch 18 is left in the open condition and no potential is appliedacross electrodes 11 and 16 in the system as seen in FIGURE 2a, thesuspended particles 28 merely assume random positions in the liquidcarrier 27. However, when switch 18 is closed thereby rendering theconductive surface 13 of electrode 11 positive with respect to the backsurface of blocking electrode 16, negatively charged particles withinthe system tend to move toward electrode 11 while any positively chargedparticles in the system would move toward blocking electrode 16. Theexistence of any positively charged particles within the system andtheir movement therein will temporarily be disregarded so as tofacilitate the explanation of the movement of negatively chargedparticles in the carrier liquid. Since the particles 28 are, in theabsence of actinic radiation, nonconductive, they come down into contactwith or closely adjacent to injecting electrode 11 and remain in thatposition under the influence of the applied electric field until theyare subjected to exposure to activating electromagnetic radiation. Ineffeet then, these particles are bound at the surface of the injectingelectrode 11 until exposure takes place because particles 28 aresufiiciently nonconductive in the suspension in their unexposedcondition to prevent the injection of positive charge from the surface13 of the electrode 11 into them. Particles bound on the surface 13 makeup the potential imaging particles for the final image to be reproducedthereon.

When photons of light such as 31 in FIGURE 2:: are produced as, forexample, by the projector which exposes the system to the image beingreproduced, they are absorbed by the photosensitive pigment in particle28b and create hole-electron pairs of charge carriers within theparticle by raising them to a conductive energy band. Since the chargecarriers are newly formed by the photons of light 31, as shown in FIGURE20, they have not had a chance to become trapped in charge traps withinthe body of the particle 28b as was the encircled negative chargecarrier. Accordingly, these newly formed charge carriers may beconsidered as mobile in nature and have been represented by unencircledplus and minus signs. Since an electric field is applied across theparticles by the potential applied across electrode 16 and conductivesurface 13 of electrode 11, the hole-electron pairs created within theseparticles are caused to separate before they can recombine, withnegative charge carriers moving towards surface 13 while positive chargecarriers move up toward electrode 16. Since the charge carrier asinitially formed are in a mobile condition, the negative charge carriersnear the pigment-electrode interface can move across the very shortdistance out of the particle 28b to the surface 13 as indicated by thesmall arrow, leaving the particle with a net positive charge. Sinceparticle 28b now carries a net positive charge, it is re pelled away bythe positive surface 13 of electrode 11 and attracted to negativeblocking electrode 16, moving as indicated by arrow 32 in FIGURE 2d.Accordingly, all particles such as 2812 on the surface 13 which areexposed to electromagnetic radiation of a wavelength to which they aresensitive (that is to say, a wavelength which will cause the formationof hole-electron pairs within the particles) move away from surface 13up to the surface of electrode 16, leaving behind those particles suchas 280 which are either not exposed at all or not exposed toelectromagnetic radiation to which they are sensitive. Particlesreaching the blocking electrode surface 16 adhere thereto since thissurface is substantially insulating and resists injection of charge fromthe particles. Consequently, if all particles in the system aresensitive to one wavelength of light or another and the system isexposed to an image with that wavelength of light, a positive image willbe formed on the surface of elect-rode 13 by the subtraction of boundparticles from its surface in exposed areas leaving behind boundparticles in unexposed areas. The system is also capable of creating aphotographically negative image on surface 16 since only particles inexposed areas move up to that surface. As particles such as 28b move upthrough the liquid carrier 27 from surface 13 towards electrode 16, itis believed that the new charge carriers enter charge carrier traps andthis has been indicated diagrammatically by showing the holes enclosedwithin circles in FIGURE 2d. Accordingly, the particle now contains onetrapped electron and two trapped holes giving it a net charge of plus 1.

As should be clear at this point in the disclosure, there are certainpreferred properties for electrodes 11 and 16. These are that electrode11 will preferably be capable of accepting injected electrons from boundparticle 28b when it is exposed to light so as to allow for a net changein the charge polarity on the particle and that electrode 16 willpreferably be a blocking electrode which is incapable of injectingelectrons into particle 281: at more than a very slow rate when it comesinto contact with the surface of the electrode 16. Obviously, if allpolarities in the system are reversed, electrode 11 will preferably becapable of accepting injected holes from bound particles upon exposureto light and electrode 16 would preferably be a blocking electrodeincapable of injecting holes into the particles at more than a very'slow rate when they come into contact with the surface of thiselectrode. In this preferred embodiment, electrode '11 may be composednot only of conventional conductive materials such as tin oxide, copper,copper iodide, gold or the like but may also include many slightlyconductive materials such as raw cellophane which are not ordinarilythought of as conductors but which are still capable of acceptinginjected charge carriers of the proper polarity under the influence ofthe applied field. Even highly insulating materials such aspolytetrafluoroet-hylene may be placed over the surface of the injectingelectrode and still be operative because charge which leaves theparticles initially bound on this surface upon exposure to light canmerely move out of the particles and remain on the insulating surfacethereby allowing the exposed particles to migrate. However, the use ofthe more conductive materials is preferred because it allows for cleanercharge separation in that charge leaving the particles upon exposeurecan move into the underlying surface and away from the particle in whichit originated. This also prevents possible charge buildup on theelectrode which might tend to diminish the inter-electrode field. On theother hand, the preferred embodiment of the blocking electrode 16 isselected so as to prevent or greatly retard the injection of electrons(or 'holes, depending upon the initial polarity of charge on theparticle) into particle 28b when it reaches the surface of thiselectrode. Accordingly, the surface of this electrode facing carrierliquid 27 in the preferred embodiment may be either an insulator or asemiconductor which will not allow for the passage of suflicient chargecarriers under the influence of the applied field to discharge theparticles finally bound to it thereby preventing particle oscillation inthe system. Even if this blocking electrode will allow for the passageof some charge carriers through it to the particles, it will still beconsidered to come within the class of preferred materials if it doesnot allow for the passage of sufficient car-riers to recharge theparticle to the opposite polarity because even a discharged particlewill tend to adhere to this blocking electrode by Van Der Waals forces.Here again, materials not coming within the preferred class may beemployed but they tend to lead the particle oscillation in the system,resulting in lower image density, poorer image resolution, imagereversal and similar deficiences, with the degree of these defic-iences,in most instances depending upon how far the material employed deviatesfrom the preferred class of materials in its electrical characteristics.Baryta paper and other suitable materials may be employed to surface theblocking electrode and may be wet on their back surfaces with tap wateror coated on these back surfaces with electrically conductive materials.Baryta paper consists of a paper coated with barium sulfate suspended ina gelatin solution. The terms blocking electrode and injecting electrodeshould be understood and interpreted in this context throughout thespecification and claims. As described in greater detail hereinafter,the system may be operated with suspensions of particles which initiallytake on a net positive charge, or a net negative charge, and even withsystems where the particles in suspension apparently take on bothpolarities of charge.

Suitable substantially insulating carrier liquids for the system includedecane, dodecane, N-tetradecane, molten paraffin, molten beeswax, SohioOdorless Solvent 3440 (a kerosene fraction available from Standard OilCompany of Ohio), and Isopar G (a long chain saturated aliphatichydrocarbon available from Humble Oil Company of New Jersey). Any othersuitable substantially insulating liquid may be used.

A wide range of voltages may be employed between the electrodes in thissystem. For good image resolution, high image density and lowbackground, it is preferred that the potential applied be such as tocreate an electrical field of at least about 300 volts per mil acrossthe imaging suspension. The applied potential necessary to attain thisfield strength will, of course, vary depending upon the interelectrodegap and on the thickness and type of blocking material used on theblocking electrode surface. For the very highest image quality, theoptimum field is at least about 2,000 volts per mil. The upper limit offield strength is limited only by the breakdown potential of thesuspension and blocking material. Fields below about 300 volts per mil,while capable of producing images, generally produce images of lowdensity and of irregular density across the image.

The field here is found by dividing the inter-electrode gap into thepotential applied between the electrodes. The field is assumed to beapplied across this gap. Thus, where the two electrodes are spaced about1 mil apart, a potential of about 300 volts applied between the blockingelectrode core and the injecting electrode surface will produce a fieldacross the suspension of about 300 volts per mil.

Depending upon the particular use to which the system is to be put, theliquid suspension 14 may contain 1, 2, 3, or even more differentparticles of various colors and having different ranges of spectralresponse. Thus, for example, in a mono-chromatic system, the particlesincluded in imaging liquid 14 and may be virtually any color in which itis desired to produce the final image such as gray, black, blue, red,yellow, etc. and the particular point or range of its spectral responseis relatively immaterial as long as it shows response in some region ofthe visible spectrum which can be matched by a convenient exposuresource. There should, however, be substantial coincidence between theprimary spectral absorption range and the primary photosensitiveresponse range of the particles to insure high photographic sensitivityin the system. In fact, in a monochromatic system, the pigment may varyin response from one with a very narrow response band all the way up toone having panchromatic response. In polychromatic systems, theparticles may be selected so that particles of different colors respondto different wavelengths in the visible spectrum, thus allowing forcolor separation. It should be noted, however, that this separation ofspectral responses of differently colored included particles is notrequired in all instances and in some cases may actually be undesirable.Thus, for example, in a monochromatic black and white system where it isdesirable to produce very intense black images, it may be preferred toproduce this result by employing two or more differently coloredpigments in the system, which when combined will produce a black image.In this latter in stance, considerable overlap and even coincidence ofthe spectral response curves of the diiferent pigments may be toleratedand may even be preferred so that all of the pigments employed in thesystem will respond in a substantially similar Way to generallyavailable light sources which are not uniformly panchromatic in theirlight output. Clearly, if a white light source is used, this overlap isnot a requirement.

Any suita'ble photosensitive particle or mixtures of such particles maybe used in carrying out the invention, regardless of Whether theparticular particle selected is organic, inorganic and is made up of oneor more components in solid solution or dispersed one in the other orWhether the particles are made up of multiple layers of differentmaterials. Typical organic pigments include: quinacridones such as:2,9-dimethyl quinacridone, 4,11-dimethyl quinacridone,3,10-dichloro-6,13-dihydro-quinacridone,2,9-dimethoXy-6,13-dihydr-quinacridone,2,4,9,1ltetrachloro-quinacridone, and solid solutions of quinacrid-onesand other compositions as described in US. Patent 3,160,510;carboxamides such as: N-2"-pyridyl-8,13-dioxodinapht'ho-(l,2-2',3)furan-6-carboxamide, N-2"-(l",3"-diazyl)-8,13-dioXodinaphtho-(1,2-2',3') furan-6- carboxamide,N-2"-(1",3",5"-triazyl-8,l3-dioxodinaphtho-(l,2-2',3)furan-6-carboxamide,anthra-(2,1,fi)-naphtho-(2,3-d)-furan-9,14-dione-7-(2'-methylphenyl)carboxamide;carboxanilides such as: 8,13-dioxodinaphtho-(l,2-2,3')-furan-6-carbox-p-methoxyanilide, 8,13-dioxodinaphtho(1,2-2',3') furan-6-carboX-p-methylanilide, 8,13 dioxodinaphtho(l,2-2',3') furan-6-carboX-Inchloroanilide, 8,13-di0xodinaphtho(1,2-2,3')-furan-6- carboX-p-cyanoanilide; triazines such as:2,4-diaminotri-azine, 2,4-di-(l-anthraquinonyl-amino)6-(1"-pyrenyl)-triazine, 2,4-di-(l-anthraquinonyl-amino-6-(1"-naphthyD-triazine,2,4-di-(l'-naphthyl-amino) 6-(l'-perylenyl)-triazine,2,4,6-tri-(1,1",1-pyrenyl) triazine; benzopyrrocolines such as:2,3-phthaloyl-7,S-benzopyrrocoline,1-cyano-2,3-phthaloyl-7,S-benzopyrrocoline, 1-cyano-2,3- phthaloyl nitro7,8-benz0pyrr0coline, 1-cyano-2,3- phthaloyl-S-acetamido 7,8benzopyrrocoline; anthraquinones such as:1,5-bis-('beta-phenylethylamino) anthraquinone, 1,5-bis-(3'-methoxypropylamino) anthraquinone, 1,5-bis (benzylamino)anthraquinone, 1,5-bis (phenylbutylamino) anthraquinone, 1,2,5,6di(C,C-diphenyl)- thiazole-anthraquinone, 4-(2'-hydr0Xypheny1methoxyamino) anthraquinone; azo compounds such as: 2,4,6-tris(N-ethyl-N-hydroxy-ethyl-p-aminophenylazo) phloroglucinol, 1,3,5,7tetrahydroxy-Z,4,6,8-tetra (N-methyl-N-hydroXyethyl-p-amino-phenylazo)naphthalene, 1,3,5-trihydroxy 2,4,6 tri(3'-nitro-N-methyl-N-hydroxymethyl-4'- aminophenylazo) benzene,3-methyl-1-phenyl-4-(3'-pyrenylazo) -2-pyrazolin-5-one, 1- (3'-pyrenylazo -2-hydroxy-3- naphthanilide, 1-(3-pyrenylazo)-2-naphthol,1-(3-pyrenylazo)-2-hydroxypyrene, 1 (3-pyrenylazo)-2-hydroxy-3-methyl-Xanthene, 2,4,6-tris (3'-pyrenylazo) phloroglucinol, 2,4,6-tris(l-phenanthenylazo) phloroglucinol, 1-(2- methoXy-5'-nitrophenylazo)-2-hydroxy-3-nitro-3-naphthanilide; salts and lakes ofcompounds derived from 9- phenylxanthene, such as:phosphotungstomolybdic lake of 3,6-bis (ethylamino)-9,2-carboxyphenylxanthenonium chloride, barium salt of 3-2-toluidine amino-6-2"-methyl-4" sulphophenyl amino-9-2"-carboxyphenyl Xanthene; phosphomolybdic lakeof 3,6-bis (ethylamino)-2,7-dimethyI-9-2'-carbethoxyphenyl Xanthenoniumchloride; dioxazines such as:2,9-dibenzoyl6,13-dichloro-tripl1enodioxazine,2,9-diacetyl-6,13-dichloro-triphenodioxazine,3,10-dibenzoylamino-2,9-diisopropoxy-6,l3-dichloro triphenodioxazine,2,9 difuroyl 6,13 dichloro-triphenodioxazine; lakes of fluorescein dyes,such as: lead lake of 2,7 dinitro- 4,5-dibromo fiuorescein, lead lake of2,4,5,7-tetrabromo fluorescein, aluminum lake of2,4,5,7-tetrabromo-10,11, 12,13-tetrachloro fluorescein; bisazocompositions such as: N,N-[1-(l-naphthylazo)2-hydroXy-8-naphthyl]adipdiamide, N,N-di- 1-( 1-naphthylazo -2-hydroxy-8-naphthylsuccindiamide,bis-4,4-(2-hydroxy-8"-N,N-diterephthalamide-l-naphthylazo)biphenyl,3,3-methoxy-4,4'-dipheny1-bis( 1"-azo-2"-hydroxy-3"-naphthanilide)pyrenes such as: 1,3,6,8-tetracyanopyrene,1,3-dicyano-6,8-dibromo-pyrene, 1,3,6,8-tetraaminopyrene,1-cyano-6-nitropyrene; phthalocyanines such as: beta-form metal-freephthalocyanine, copper phthalocyanine, tetrachloro phthalocyanine, thex-form of metal-free phthalocyanine as described in copendingapplication Ser. No. 505,723, filed Oct. 29, 1965; metal salts and lakesof azo dyes, such as: calcium lake of 6-bromo-1(1'-sulfo-2-naphthylazo)-2-naphthol, barium salt of6-cyano-1(l-sulfo-2-naphthylazo)-2-naphthol, calcium lake of1-(2-azonaphthaliene-1'-sulfonic acid)-2-naphthol, calcium lake of1-(4-ethyl-5-chloroazobenzene-2'-sulfonic acid)-2-hydroxy-3-naphthoicacid; and mixtures thereof.

Typical inorganic compositions include cadmium sulfide, cadmiumsulfoselenide, zinc oxide, zinc sulfide, sulphur selenium, mercuricsulfide, lead oxide, lead sulfide, cadmium selenide titanium dioxide,indium trioxide and the like. In addition to the aforementioned organicpigments other organic materials which may be employed in the particlesinclude polyvinylcarbazole;

2,4-bis(4,4'-diethyl-aminophenyl)-1,3 ,4oxidiazole;

N-isopropylcarbazole;

polyvinylanthracene;

triphenylpyrrol 4,5 -di phenylimidazolidinone 4,5 -diphenylimidazolidinone;

4,5 -diphenylimidazolidinethione;

4,5 -bis- 4-amino-phenyl) -imidazolidinone 1,2,5,6-tetraazacyc1ooctatetraene- 2,4,6,8

3,4-di- (4'-methoxyphenyl) -7,8-diphenyll,2,5,6-tetraaza-cyclooctatetraene 2,4,6,8)

3,4-di(4'-phenoXy-phenyl)-7,8-diphenyl-1,2,5,6-tetraaza-cyclooctatetraene-(2,4,6,8);

3,4,7 ,8-tetramethoxy- 1,2,5 ,6-tetraaza-cyclooctatetraene- Z-mercaptobenzthiazole;

2-phenyl-4-alpha-naphthylidene-oxazolone;

2-phenyl-4-diphenyl-idene-oxazolone;

2-phenyl-4-p-methoxyb enzylidene-oxazolone;

6-hydroxy-2-phenyl(p-dirnethyl-amino phenyl)- benzofurane;

6-hydroxy-2,3 -di (p-methoxyphenyl) -benzofurane;

2,3 ,5 ,6-tetrap-methoxyphenyl) -fuoro- (3,2f

benzofurane;

4-dimethylamino-b enzylidenebenzhydrazide;

4-dimethyl-aminobenzylideneisonicotinic acid hydrazide;

turfurylidene- 2) -4'-dimethylamino-benzhydrazide;

S-benzilidene-amino-acenaphthene-B-benzylideneamino-carb azole;

( 4-N,N-dimethylamino-benzylidene) -p-N,N-dimethylaminoaniline;

(Z-nitro-benzylidene -p-bromo-aniline;

N ,N-dimethyl-N- (2-nitro-4-cyano-benzylidene) -pphenylene-diamine;

2,4-diphenyl-quinazoline;

2- (4'-amin0phenyl -4-phenyl-quinazoline;

2-phenyl-4- 4-dimethyl-amino-phenyl -7-methoxyquinazoline1,3-diphenyl-tetrahydroimidazole;

1, 3-di- 4'-chlorophenyl -tetra-hydroimidazole;

1,3-diphenyl-2-4-dimethyl aminophenyl)-tetrahydroimidazole;

1,3-di- (p-tolyl) -2- [quinolyl- 2') -tetrahydroimidazole;

3- 4-di-methylamino-phenyl) -5-'( 4"-methoxy-phenyl'6-phenyl-1,2,4-triazine;

3-pyridil- (4 -5- (4-dimethylaminophenyl -6-phenyl- 1,2,4-triaziue;

3 -(4-amino-phenyl) 5,6-di-phenyl-1,2,4-triazine;

2,5 -bis [4-amino-phenyl-( l ]-1,2,3-triazole;

2,5-bis[4'-(N-ethyl-N-acetylamino)-phenyl-(1) 1,3 ,4-triazole;

1,5 -di phenyl-3 -methyl-pyrazoline;

1,3,4,5-tetraphenyl-pyrazoline;

1-phenyl-3 (p-methoxy styryl (p-methoxy-phenyl pyrazoline;

1-methyl-2- 3',4'-dihydroxy-methylene-phenyl) benzimidazole;

2- (4'-dimethylaminephenyl) -benzoxazole;

2- 4'-methoxyphenyl -b enzthiazole;

2,5-bis[p-amino-phenyl-( 1) ]-1,3,4-oxidiazole;

4,5 -diphenyl-imid azolone;

3-amino-carb azole;

copolymers and mixtures thereof.

Other materials include organic donor-acceptor (Lewis acid-Lewis base)charge-transfer complexes made up of aromatic donor resins such asphenolaldehyde resins, phenoxies, epoxies, polycarbonates, urethanes,styrene or the like complexed with electron acceptors such as 2,4,7-trinitro-9-fluorenone; 2,4,5,7-tetranitro-9-fluorenone; picric acid;1,3,5-trinitro benzene; chloranil; 2,5dichl0r0- benzoqulnone;anthraquinone-Z-carboxylic acid, 4nitrophenol; maleic anhydride; metalhalides of the metals and metalloids of Groups I-B and II-V-III of thePeriodic Table including for example, aluminum chloride, zinc chloride,ferric chloride, magnesium chloride, calcium iodide, strontium bromide,chromic bromide, arsenic triiodide, magnesium bromide, stannous chlorideetc.; boron halides, such as boron trifluorides; ketones such asbenzophenone and anisil, mineral acids such as sulfuric acid; organiccarboxylic acids such as acetic acid and maleic acid, succinic acid,citroconic acid, sulphonic acid, such as 4-toluene sulphonic acid andmixtures thereof. In addition to the charge transfer complexes, it is tobe noted that many other of the above materials may be furthersensitized by the charge transfer complexing technique and that many ofthese materials may be dye-sensitized to narrow, broaden or heightentheir spectral response curves.

As stated above, any suitable particle structure may be employed.Typical particles include those which are made up of only the purephotosensitive material or a sensitized form thereof, solid solutions ordispersions of the photosensitive material in a matrix such asthermoplastic or thermosetting resins, copolymers of photosensitivepigments and organic monomers, multilayers of particles in which thephotosensitive material is included in one of the layers and where otherlayers provide light filtering action in an outer layer or a fusable orsolvent softenable core of resin or a core of liquid such as dye orother marking material or a core of one photosensitive material coatedwith an overlayer of another photosensitive material to achievebroadened spectral response. Other photosensitive structures includesolutions, dispersions, or copolymers of one photosensitive material inanother with or without other photosensitively inert materials. Whilethe above structural and compositional variations are useful, it ispreferred that each particle be primarily composed of an electricallyphotosensitive pigment, such as those listed above, wherein the pigmentis both the primary electrically photosensitive ingredient and theprimary colorant for the particle. These particles have been found togive optimum photographic sensitivity and highest overall image qualityin addition to being simple and economical to prepare. Of course, it mayoften be desirable to include other ingredients, such as spectral orelectrical sensitizers or secondary colorants and secondary electricallyphotosensitive materials.

Regardless of whether the system is employed to reproduce amonochromatic or a polychromatic image, it is desirable to use pigmentparticles which are relatively small in size because smaller particlesproduce better and more stable pigment dispersions in the liquid carrierand, in addition, are capable of producing images of higher resolutionthat would be possible with particles of larger sizes. In general, bestresults have been obtained with particles having an average diameter ofup to about 5 microns. While satisfactory images may be obtained withlarger particles, the images tend to be splotchy in appearance and tohave low density. For optimum image density and uniformity of densityacross the image, particles having a diameter up to about 1 micronshould be used. Even where the pigments are not commercially availablein small particle sizes, the particle size may be reduced byconventional techniques such as extended ball milling or the like. Whenthe particles are suspended in the liquid carrier, they may take on anet electrostatic charge so that they may be attracted towards one ofthe electrodes in the system depending upon the polarity of this chargewith respect to that of the electrodes. It is not necessary that theparticles take on only one polarity of charge but instead the particlesmay be attracted to both electrodes. Some of the particles in thesuspension initially move towards the injecting electrode while othersmove towards the blocking electrode with this type of system; however,this particle migration takes place uniformly over the whole areacovered by the two electrodes and the effect of imagewise,exposure-induced migration is superimposed upon it. Clearly then, theapparent bipolarity of the suspensions in no way affects the imagingcapability of the system except for the fact that it subtracts some ofthe particles uniformly from the system before imagewise modulation ofthe particle migration takes place. In other words, the above behaviorcauses a portion of the suspended particles to be removed from thesystem as potential image-formers. The effective subtraction of some ofthese particles as potential image formers in the system is easilyovercome by merely forming an initial suspension of particles containinga sufiiciently high particle concentration so that the system is stillcapable of producing intense images. It also has been found that withsome suspensions of this type, either polarity of potential may beapplied to the electrodes during imaging. Although some of thephotosensitive pigment materials used in this invention may he used inconventional dry modes of operation, it is believed that a differenttype of photoresponsive mechanism is involved because it has generallybeen found that spectral response of the materials is much narrower andtheir sensitivity is much higher when they are used in the liquidcarrier structure of this invention. Also, in dry systems Van der Waalsforces have a serious detrimental effect on imaging with particlessmaller than about 10 microns. Surprisingly, in the liquid carrier ofthe present invention particles smaller than 1 micron may be usedwithout significant interference due to Van der Waals forces.

The addition of small amounts (generally ranging from .5 to 5 molpercent) of electron donors or acceptors to the suspensions with thechoice depending upon whether the particles attracted to the injectingelectrode are positive or negative respectively has imparted significantincreases in system photosensitivity as described in the examples. Thiseffect is believed to be caused either by the scavening of free chargecarriers from the system or from an initial charge build up on thesurface of the particles. For further details of electricallysensitizing this system, see copending application Ser. No. 566,846,filed July 21, 1966.

As stated above, once the particle image is formed on one of theelectrodes, it may be fixed thereon as by spraying a binder on it,laminating an overlay on it or by including a binder in solution in theliquid suspending medium. In most instances, however, it will be foundpreferable to transfer the image from the electrode and fix it onanother surface so that the electrode may be reused. Such a transferstep may be carried out by adhesive pickoif with an adhesive tape suchas Scotch brand cellophane tape or preferably, by electrostatic fieldtransfer. Electrostatic transfer may, for example, be carried out bycarrying out the imaging procedure described in connection with FIGURE1a and then passing a second roller over the particle image formed onelectrode 11 held at a potential opposite in polarity to that of thefirst electrode. If the second electrode roller is covered with a barytapaper sleeve, this paper sleeve will pick up the complete image as theelectrode rolls over it. For further details on this transfer system,see copending application Ser. No. 542,050, filed Apr. 12, 1966.

Although various electrode spacings may be employed, spacings of lessthan 1 mil and extending down even to the point where the electrodes arepressed together as in the case of the roller electrode of FIGURE 1aconstitute a particularly preferred form of the invention in that theyproduce better image density and background than is produced with widerspacings. Wider spacings tend to result in very high backgrounddeposition. Optimum density and background are generally obtained withan inter-electrode spacing of about 0.2 mils (where the electrodes arepressed together).

Any suitable proportion of electrically photosensitive particles tocarrier liquid may be used. It is preferred that from about 2 to about10 weight percent particles be used for good balance between high imagedensity and low background, consistent with particle economy. Less than2 wt. percent particles tends to cause Streaky images, While over 10 wt.percent particles tends to cause mottling in the image. Optimum imagequality has been obtained with from about 5 to 6 weight percentparticles.

The layer of the imaging suspension may be coated onto either electrodebefore imaging. Generally, the layer should have a thickness about 2mils greater than the inter-electrode spacing to insure that bothelectrodes uniformly contact the suspension. Still greater relativelayer thicknesses may be used, since the excess is merely squeezed outas the electrodes are brought into place. However, this excess is notneeded for imaging and is uneconomical. Where the inter-electrodespacing is up to 1 mil, optimum results are obtained with a suspensionlayer thickness of 0.5 to 3 mils.

Description of preferred embodiments The following illustrative examplesare given to enable those skilled in the art to more clearly understandand practice the invention. Parts and percentages are by weight unlessotherwise indicated. These examples may be considered to illustratepreferred embodiments of the present invention.

EXAMPLES I-XXIX Each of these examples is carried out in an apparatus ofthe general type schematically illustrated in FIGURE 1a with the imagingmix coated on a NESA glass substrate through which exposure is made. TheNESA glass surface is connected in series with a switch, a potentialsource, and the conductive center of a roller having a coating of barytapaper on its surface. The roller is approximately 2.5 inches in diameterand is moved across the plate surface at about 1.5 centimeters persecond. The plate employed is roughly 3 inches square and is exposedwith a light intensity of about 1800 foot-candles. About 7 percent byWeight of the indicated finely divided photosensitive material in eachexample is suspended in Sohio Odorless Solvent 3440. During imaging,unless otherwise indicated, a positive potential of about 2500 volts isimposed on the core of the roller. The gap between the baryta papersurface and the NESA glass surface is about 1 ml. With all pigmentswhich are received commercially with a relatively large particle size,the particles are ground in a ball mill for about 48 hours to reducetheir size to an average diameter of less than 1 micron. Exposure ismade with a 3200 K. lamp through a 0.30 neutral density step wedgefilter to measure the sensitivity of the suspension to white light andthen Wratton filters 29, 61 and 47b are individually superimposed overthe light source in separate runs to measure the sensitivity of thesuspension to red, green, and blue light, respectively. The relativesensitivity response figures obtained for the suspension are tabulatedin Table I below. The sensitivity figures are derived from the number ofsteps step wedge filter which are discernible in the images made throughthe filter, Thus, Where one step is visible in the image, sensitivity isone; where two are visible, it is two; where three are visible, it isfour; where four are visible, it is eight, etc.

In addition to the sensitivity tests, each of the compositions listedbelow is suspended in the carrier liquid and exposed to a conventionalblack-and-white transparency containing line copy images using whitelight. Each of the compositions listed below produces an image of goodquality, with a positive image conforming to the original formed on theNESA glass surface and a negative image formed on the roller surface.

In these examples, the particles are homogeneous, each made up of asingle composition as follows:

Example I.Locarno Red X-l686, l-(4'-methyl-5'-chloroazobenzene-2'-sulfonic acid) 2 hydroxy-3-naphthoic acid, C.I. No.15865, available from American Cyanamid;

Example II.-Watchung Red B, a barium salt of 1(4'-methyl-5-chloroazobenzene-2'-sulfonic acid)-2-hydroxy- 3-naphthoic acid,OI. No. 15865, available from E. I. du Pont de Nemours & Co.;

Example III.Permagen 'Red L toner 51-500, 1-(4'-methyl-5'-ch1oroazobenzene-2-sulfonic acid)-2-hydroxy- 3-naphthoic acid,C.I. No. 15 865, available from Collway Colors;

Example IV.Naphthol Red B, 1-(2'-methoxy-5-nitrophenylazo)-2hydroxy-3"-nitro-3-naphtlranilide, C.I. No. 12355, available fromCollway Colors;

Example V.Duol Carmen, the calcium lake of 1-(4-methylazobenzene-Z'-sulfonic acid) 2-hydroxy-3-naphthoic acid, C.I. No.15850, available from E. I. du Pont de Nemours & Co.;

Example VI.Bonadur Red B, an insolubilized azo dye available fromCollway Colors. This pigment is a dye described in (3.1. No. 15865 withhydrogen substituted for the sodium in the compound to insolubilize it.

Example VII.Calcium Lithol Red, the calcium lake of1-(2-azonaphthalene-1'-sulfonic acid)-2-naphthol, C.I. No. 15630,available from Collway Colors.

Example V-HL-Indofast Double Scarlet Toner, a pyranthrone-type pigment,available from Harmon Colors. This pigment is a polynuclear aromatichaving the following structure:

Br I

Example lX.Quindo magenta lRV-6803, a quinacridone-type pigment,available from Harmon Colors, having the following structure:

J S 6 H Example X.--Indofast Brilliant Scarlet Toner, 3,4,9, 10 bis[N,N-(p-methoxyphenyl)-imido]-perylene, 0.1. No. 71140, available fromHarmon Colors.

13 Example XI.-Indofast Red -MV-6606, a thioindoxyletype pigment,available from Harmon Colors, having the following structure:(dichlorothioindigo) Example XII-Vulcan Fast Red BBE Toner 35-2201,3,3-dimethoxy-4,4'-biphenyl bis(1" phenyl3"-methyl-4"-azo-2"-pyrazolin-5"-one), C.-I. No. 21200, available from CollwayColors.

Example XIII.-Pyrazolone Red B Toner, C.I. No. 21120, available fromCollway Colors, having the following structure:

Example XIV.Cyan Blue GTNF, the beta form of copper phthalocyanine,(3.1. No. 74160, available from Collway Colors.

Example XV.Cyan =Blue XR, the alpha form of copper phthalocyanine,available from Collway Colors.

Example XVI.Monolite Fast Blue GS, the alpha form of metal-freephthalocyanine, CI. 74100, available from Arnold Hoffman Company.

Example XVII.-Monolite Fast Blue GS with about 3 mol percent2,4,7-trinitro-9 fiuoroenone added to the suspension.

Example XVIII.Monolite Fast Blue GS with about 2 mol percentbenzonitrile added to the suspension.

Example X-IX.--Monolite Fast Blue GS treated by milling ino-dichlorobenzene.

Example XX.-Meth-yl Violet, a phosphotungstomolybdic acid lake of4-(N,N',N'-trimethylanilino)methylene-N",N"dimethylanilinium chloride,0.1. No. 42535, available from Collway Colors.

Example XXI.Indofast Violet Lake, dich1oro-9,18- isoviolanthrone, C.I.No. 60010, available from Harmon Colors.

Example XXI'I.Diane Blue,3,3'-methoxy-4,4'-diphenyl-bis(1"-azo-2"-hydroxy 3"-naphthanilide),C.-'I. No. 21180, available from Harmon Colors.

Example XXIII.-A polychloro substituted copper phthalocyanine, Cl. No.74260, available from Imperial Color and Chemical Company.

Example XXIV.-Indanthrene Brilliant Orange RK,4,10-dibromo-6,IZ-anthanthrone, C.I. No. 59300, available from GeneralDyestuffs.

Example XXV.--Algol Yellow GC,1,2,5,6-di(C,C'-diphenyl)-thiazole-anthroquinone, C.I. No. 67300,available from General Dyestuffs.

TABLE 1 Roller Example Electrode Blue Green Red White Polarity I{POSii2lVB 2 8 0 32 Negative... 2 8 0 32 II Positive... 1 4 0 32 In IPositive... 8 32 0 64 lNegative 8 32 0 64 IV. Positive. 1 4 0 16 V 4 161 64 VI- 2 8 2 64 VII 1 4 0 16 VIII 4 8 0 32 IX 2 16 0 128 X 32 64 0 128XI 1% 38 8 32 2 64 XII '{I1;Ieg %tive.. 3 1g 0 32 us ive. 0 4 XIII"{Negative.. 1 4 0 16 XIV Positive... 1 1 16 32 d0 1 4 16 32 1 8 32 64 216 64 128 4 32 128 256 1 4 32 64 0 1 1 8 0 8 0 32 0 1 8 16 0 0 16 32 416 0 32 2 0 0 8 32 8 0 128 16 4 0 64 0 8 16 32 0 20 EXAMPLES XXX-XXXVThese examples compare image quality obtained with varyinginter-electrode spacing. Each of these examples is carried out in anapparatus of the general type described in Example I-XXIX, above. Foreach example, a positive potential of about 2,000 volts is imposed onthe core of the blocking electrode roller. The original used is ablack-and-white line copy transparency. In each example, the imagingsuspension consist of about 8 parts by weight of particles of MonliteFast Blue GS, alpha form metal-free phthalocyanine, available from theArnold Hoffman Company, having an average particle diameter of less than1 micron, dispersed in about 100 parts Sohio Odorless Solvent 3440. Thesuspension layer coated on the injecting electrode before imaging has athickness 2 mils greater than the interelectrode spacing. The resultsobtained with various interelectrode spacings are as follows:

Example XXX-The inter-electrode gap is about 20 mils, so that theapplied electric field is just under 100 volts per mil. The imageproduced is of very low quality with low density and very highbackground.

Example XXXI.-The inter-electrode spacing is about mils, so that theapplied electric field is just under 200 volts per mil. The image is oflow quality with high background; image quality is only slightly greaterthan in Example XXX.

Example XXXII.-The inter-electrode spacing is about 5 mils, so that theelectric field applied is just under 400 volts per mil. An image ofsatisfactory quality is obtained, with good density but high background.

Example XXXIIL-The inter-electrode spacing is about 1 mil, so that theapplied electric field is just under 2,000 volts per mil. The resultingimage is of excellent quality with high image density and lowbackground.

Example XXXIV.T-he inter-electrode spacing is about 0.5 mil, so that theapplied electric field is just under 4,000 volts per mil. The resultingimage is of excellent quality with excellent density and low background.

Example XXXV.--The inter-electrode spacing is about 0.2 mil. At thisspacing, the roller is pressed very tightly against the imagingsuspension. With the imaging suspension in place this is approximatelythe minimum spacing obtainable. At this spacing, the applied electricfield is about 10,000 volts per mil. The resulting image is of excellentquality with excellent density and low background.

EXAMPLES XXXVI-XLI Each of these examples is carried out in an apparatusof the general type described in Examples I-XXIX, above. These examplescompare images obtained with varying potential applied to the suspensionbetween the electrodes. In each of these examples, the polarity of theapplied potential is positive. The original used is a black and-whiteline copy transparency. A gap of about 1 mil is maintained between theelectrodes throughout these examples. The electrically photosensitiveparticles consist of Al-gol Yellow 60,1,2,5,6-di(C,C'-diphenyl)-thiazole-anthraquinone, 0.1. No. 67300,available from General Dyestuffs. About 8 parts by weight of thismaterial having an average particle size of less than 1 micron isdispersed in about 100 parts Isopar G. A layer of this suspension havinga thickness of about 3 mils is coated on the injecting electrodeimmediately before imaging. The results obtained with differentpotentials are as follows:

Example XXXVI.The applied potential is about 50 volts, resulting in anelectric field of just under 50 volts per mil across the imagingsuspension. An image of very low quality with low density and a splotchyappearance results.

Example XXXVII.--'Ihe applied potential is about 100 volts, so that theelectric field across the suspension is just under 100 volts per mil.The image resulting is of low quality with low density and is irregularin coverage.

Example )Q(XVIlI.The applied potential is about 300 volts so that theelectric field applied is just under 300 volts per mil. An image ofsatisfactory quality is obtained with low density and some irregularityin density across the image.

Example XXXIX.--The potential applied is about 500 volts so that theelectric field across the suspension is 16 just under 500 volts per mil.An image of good quality results, with good density and uniformity.

Example XL.The applied potential is about 1,000 volts so that theelectric field across the suspension is just under 1,000 volts per mil.An image of excellent quality with good density and uniformity results.

Example XLI.--The applied potential is about 300 volts so that theelectric field across the suspension is just under 3,000 volts per mil.An image of excellent quality with excellent density and uniformityresults.

EXAMPLES XLII-XLV Each of these examples is carried out in an apparatusof the generaltype illustrated in FIGURE 1a and described in ExamplesI-XXIX, above. These examples are intended to illustrate the efiect ofvarying particle size on image quality. The original image to bereproduced here is a black and White transparency containing highresolution line copy material. The inter-electrode gap is about 1 miland a positive potential of about 2,000 volts is applied to the blockingelectrode core. The photosensitive particles consist of Watchung Red B,a barium salt of l-(4'-methyl-5'-chloroazobenzene2'-sulfonic acid)-2-hydroxy-3-naphthoic acid, 0.1. No. 15865, available from E. I. du Pontde Nemours & Co. About 8 parts by weight of this material in finelydivided form is dispersed in about parts Sohio Odorless Solvent 3440. Alayer of this suspension having a thickness of about 3 mils is coatedonto the injecting electrode surface immediately before imaging. Thequality of the images obtained with varying particle sizes is asfollows:

Example XLII.-The particles have an average diameter of about 0.5micron. The resulting image is of excellent quality with uniformcoverage and high density.

Example XLIII.--The average particle size is about 1 micron. Theresulting image is of high quality with high density and highresolution, the resolution being nearly as good as in Example XLII.

Example XLIV.The average particle size is about 5 microns. The resultingimage is of good quality with high density but noticeable fall off inimage uniformity.

Example XLV.The average particle size is about 20 microns. The image isof good quality with good density but very poor density uniformityacross the image.

EXAMPLES XLVIL Each of these examples is carried out in an apparatus ofthe general type schematically illustrated in FIGURE 1a with the imagingmix coated on a NESA glass substrate through which exposure is made.These examples are intended to illustrate the effect of varying theconcentration of photosensitive particles in the carrier liquid. Foreach example, a positive potential of about 2,000 volts is imposed onthe core of the blocking electrode roller. The original used is ablack-and-white line copy transparency. The suspension layer is coatedonto the injecting electrode surface before imaging to a thickness ofabout 2 mils. The inter-electrode gap is about 0.8 mil. The imagingsuspension consists of varying amounts of particles of the beta form ofmetal-free phthalocyanine having an average particle diameter of lessthan 1 micron, dispersed in Sohio Odorless Solvent 3440. The resultsobtained with various concentrations of particles in carrier liquid areas follows:

Example XLVI.About 0.5 part by weight of the photo-sensitive particlesare dispersed in about 100 parts by weight carrier liquid. The resultingimage is of low quality, with very low density and a streaky appearance.

Example XLVII.About 2 parts by weight of the photo-sensitive particlesare dispersed in about 100 parts carrier liquid. An image of goodquality with good density results.

Example XLVIII.About 5 parts by weight of the photo-sensitive particlesare dispersed in about 100 parts of the carrier liquid. The image is ofexcellent quality, with high density and excellent uniformity of densityacross the image.

Example XLIX.About 10 parts by weight of the photo-sensitive particlesare dispersed in about 100 parts by weight of the carrier liquid. Theimage resulting is of good quality with good density across the image.

Example L.About 3 parts by weight of the imaging particles are dispersedin about 100 parts by weight of the carrier liquid. The resulting imageis of good quality but density is irregular across the image giving amottled appearance.

EXAMPLE LI A solution is prepared by dissolving about 5 parts by weightAmberol ST-137-X, a non-reactive unmodified 100 percentphenol-formaldehyde resin, available from Rohm and Haas Company, in asolvent mixture consisting of about 25 parts acetone and about 20 partstoluene. To this solution is added about 1 part 2,4,7-trinitro-9-fluorenone. The mixture is stirred until solution of the materials iscomplete. This solution is spray dried to form particles having anaverage diameter of about 1 micron by the process described, forexample, in copending application Ser. No. 380,080, filed July 2, 1964.About 8 parts by weight of the resulting particles is dispersed in about100 parts Sohio Odorless Solvent 3440. This dispersion is coated ontothe NESA glass substrate of a device of the sort schematically shown inFIGURE 1a, to a layer thickness of about 2 microns. A positive potentialof about 3,000 volts is imposed on the core of the blocking electroderoller. An inter-electrode gap of about 0.5 mil is used. The originalused is a black-andwhite line copy transparency. The image resulting isof satisfactory quality, with satisfactory density but high background.It is apparent that these particles have low sensitivity.

EXAMPLE LII About 10 parts of Bakelite Polysulfone P1700, available fromthe Union Carbide Corporation, is dissolved in about 200 partsdichloromethane. To this solution is added a solution of about 3 parts2,4,7-trinitro-9-fiuorenone in about 50 parts cyclohexanone. To thesolution is then added about 0.3 part Rhodamine B, a green dye,9-(o-carboxyphenyl -6- diethylamino -3-xanthene-3- ylidene-diethylchloride, available from E. I. du Pont de Nemours and Company. Thesolution is stirred to insure complete mixture of the ingredients. About5 parts of very finely divided zinc oxide (having an average particlediameter of about 0.1 micron) is dispersed in this solution. Thisdispersion is then spray dried to form particles having an averagediameter of about 2 microns. About 5 parts by weight of these particlesis dispersed in about 100 parts decane. This dispersion is then coatedonto the NESA glass surface of an imaging device of the sort shown inFIGURE la to a layer thickness of about 2 mils. The inter-electrode gapis set at about 1 mil and a positive potential of about 10,000 volts isapplied to the blocking electrode core. The original image to bereproduced here is a black and white transparency containing line copyimages. The resulting image is of satisfactory quality with low densityand moderate background. It is apparent that these particles have lowsensitivity.

Although specific components and proportions have been described in theabove examples, other materials, as listed above, may be used withsimilar results, where suitable. In addition, other materials may beadded to the electrically photosensitive particles, to the imagingsuspension, or to either electrode to synergize, enhance, or otherwisemodify their properties. For example, the pigment compositions of thisinvention may be dye-sensitized or electrically sensitized if desired,or may be mixed with other photosensitive materials, both organic andinorganic.

Other modifications and ramifications of the present invention willoccur to those skilled in the art upon a reading of the presentdisclosure. These are intended to be included within the scope of thisinvention.

What is claimed is:

1. The method of photoelectrophoretic imaging comprising subjecting alayer of a suspension to an applied electric field between a pair ofelectrodes, at least one of which is partially transparent, saidelectric field having a field strength of at least about 300 volts permil, said suspension comprising a plurality of finely divided particlesin a carrier liquid, each of said particles comprising an electricallyphotosensitive pigment, said pigment being both the primary electricallyphotosensitive ingredient and the primary colorant for said particle,and substantially simultaneously exposing said suspension to an imagethrough said transparent electrode with a source of electromagneticradiation, whereby an image is formed.

2. The method of claim 1 wherein said electrodes are brought into andout of an inter-electrode spacing of up to about 1 mil while saidelectric field application and said exposure continue.

3. The method of claim 1 wherein said electrodes are brought into andout of an inter-electrode spacing of about 0.2 mils while said electricfield application and said exposure continue.

4. The method of claim 1 wherein said particles have an average size upto about 5 microns.

5. The method of claim 1 wherein said particles have an average size ofup to about 1 micron.

6. The method of claim 1 wherein said carrier liquid is a substantiallyinsulating hydrocarbon liquid.

7. The method of claim 1 wherein from about 2 to about 10 parts byweight of said particles are dispersed in about parts by weight of saidcarrier liquid.

8. The method of claim 1 wherein from about 5 to about 6 parts by weightof said particles are dispersed in about 100 parts by Weight of saidcarrier liquid.

9. The method of claim 1 wherein a film-forming binder is dissolved insaid carrier liquid and including the step of evaporating said carrierliquid from said image.

10. The method of claim 1 further including the step of overcoating theimage for-med on said electrode.

11. The method of claim 1 including the further step of transferringsaid image from at least one of said electrodes to the surface of atransfer member.

12. The method of claim 1 wherein at least one of said electrodes has ablocking surface.

13. The method of claim 1 wherein said electric field has a fieldstrength of at least about 2,000 volts per mil.

14. The method of claim 1 wherein said particles have an averagediameter of up to about 5 microns.

15. The method of claim 1 wherein said particles have an averagediameter of up to about 1 micron.

16. The method of claim 1 wherein said electrodes are brought into andout of an inter-electrode spacing of up to about 1 mil while saidelectric field application and said exposure continue, and wherein saidsuspension is coated onto the surface of one of said electrodes to athickness of from about 2 to 3 mils before imaging.

17. The method of photoelectrophoretic imaging comprising applying alayer of a suspension onto an electrode, said suspension comprising aplurality of finely divided particles in a carrier liquid, each of saidparticles comprising an electrically photosensitive pigment, saidpigment being both the primary electrically photosensitive ingredientand the primary colorant for said particle, bringing a second electrodeinto an inter-electrode spacing of up to about 1 mil, at least one ofsaid electrodes being partially transparent, applying an electric fieldacross said suspension between said electrodes, said potential being atleast about 300 volts, and substantially simultaneously exposing saidsuspension to an image through said transparent electrode with a sourceof activating electromagnetic radiation, whereby an image is formed.

18. The method of claim 17 wherein said electrodes are brought into andout of an inter-electrode spacing of up to about 1 mil while saidelectric field application and said exposure continue.

19. The method of claim 17 wherein said electrodes are brought into andout of an inter-electrode spacing of about 0.2 mils while said electricfield application and said exposure continue.

20. The method of claim 17 wherein said particles have an average sizeup to about microns.

21. The method of claim 17 wherein said particles have an average sizeof up to about 1 micron.

22. The method of claim 17 wherein said carrier liquid is asubstantially insulating hydrocarbon liquid.

23. The method of claim 17 wherein from about 2 to about parts by weightof said particles are dispersed in about 100 parts by weight of saidcarrier liquid.

24. The method of claim 17 wherein from about 5 to about 6 parts byweight of said particles are dispersed in about 100 parts by weight ofsaid carrier liquid.

25. The method of claim 17 wherein a film-forming binder is dissolved insaid carrier liquid and including the step of evaporating said carrierliquid from said image.

26. The method of claim 17 further including the step of overcoating theimage formed on said electrode.

27. The method of claim 17 including the further step of transferringsaid image from at least one of said electrodes to the surface of atransfer member.

28. The method of claim 17 wherein at least one of said electrodes has ablocking surface.

29. The method of claim 17 wherein said electric field has a fieldstrength of at least about 2,000 volts per mil.

30. The method of claim 17 wherein said particles have an averagediameter of up to about 5 microns.

31. The method of claim 17 wherein said particles have an averagediameter of up to about 1 micron.

32. The method of claim 17 wherein said electrodes are brought into andout of an inter-electrode spacing of up to about 1 mil while saidelectric field application and said exposure continue, and wherein saidsuspension is coated onto the surface of one of said electrodes to athickness of from about 2 to 3 mils before imaging.

References Cited UNITED STATES PATENTS 2,758,939 8/1956 Sugarman 961.4 X2,839,400 6/ 1958 Moncriefi-Yeates 96-1.4 2,940,847 -6/ 1960 Kaprelian96-1 3,058,914 10/1962 Metcalfe et al 252-62.l 3,145,156 8/1964 Oster204- 3,301,772 1/1967 Viro 204-2 NORMAN G. TORCHIN, Primary Examiner. C.E. VAN HORN, Assistant Examiner.

